The present invention relates generally to monitoring phase transitions in materials and, more particularly, to a method for using second harmonic generation (SHG) to identify and probe phase transitions in polymorphic materials.
By providing the appropriate thermal or mechanical input, phase transitions may be induced in materials. Phase transitions are generally identified by monitoring changes in a physical property or properties of the material during thermal and/or mechanical input. The elucidation of the phase transition behavior a material is important in understanding the properties of the material. Thus, the development of methods for identifying and probing phase transitions in materials is of great interest.
Many materials, such as organic and inorganic compounds and polymers, exhibit polymorphism; they can exist in more than one crystallographically distinct crystalline phase. These crystalline phases, known as polymorphs, can be centrosymmetric phases that have inversion symmetry, or non-centrosymmetric crystalline phases that do not have inversion symmetry. Although the individual polymorphs of a polymorphic material have the same chemical composition, they can have significantly different physical properties. For example, four distinct crystalline phases have been identified for the energetic organic compound octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). While the beta phase of HMX (xcex2-HMX) is relatively insensitive to changes in temperature and pressure, explosive decomposition may be induced in the delta phase of HMX (xcex4-HMX) with thermal and/or shock input.
Many important biologically active compounds exhibiting pharmaceutical activity, herein referred to as pharmaceuticals, are polymorphic; these include the anti-diabetic drug tolbutamide, the antibiotic chloramphenicol, and the selective estrogen response modulator tamoxifen. The phase behavior and physical properties of polymorphic pharmaceuticals must be thoroughly understood, since the properties of the various polymorphs of a pharmaceutical may different. For example, individual polymorphs of a polymorphic pharmaceutical can exhibit different rates of dissolution, which can affect their concentration in body tissues and therefore their effectiveness.
The development of crystallization procedures to induce the formation of a desired polymorph is usually expensive and time consuming. Optimizing these procedures usually involves determining operating temperatures, operating pressures, rates of heating or cooling, solvents, concentrations of materials, and other parameters. Rapid non-invasive, in-situ dynamic monitoring during crystallization can provide important information regarding the formation of desired and undesired polymorphs of polymorphic materials. While Raman spectroscopy has been used for the detection of polymorphic transitions, system costs are prohibitively expensive at the laboratory scale, and will likely limit use on the industrial scale.
It is extremely important to understand the phase behavior of polymorphic materials. Clearly, a rapid and sensitive method for identifying and probing the phase transitions of polymorphic materials is highly desirable.
Therefore, an object of the present invention is a rapid and highly sensitive method for identifying and probing phase transitions of polymorphic materials.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the invention includes a method for identifying and probing phase transitions in materials. A polymorphic material capable of existing in at least one non-centrosymmetric phase is interrogated by a beam of laser light at a chosen wavelength and frequency. A phase transition is induced in the material whilst it is interrogated. The intensity of light scattered by the material and having a wavelength equal to one half the wavelength of the interrogating laser light is detected. If the phase transition results in the production of a noncentrosymmetric phase, the intensity of this scattered light increases. If the phase transition results in the disappearance of a non-centrosymmetric phase, the intensity of this scattered light decreases.